Process for removing sulfur moieties from Claus tail-gas

Abstract

Claus tail-gas containing H 2 S and SO 2 in a mol ratio of 2:1 is passed in contact with a catalyst-adsorbent bed (19A) at a temperature of 135° to 150° C. and a space velocity of 50 to 1500 h -1 whereby elemental sulfur produced by the reaction of H 2 S and SO 2 is deposited upon and/or adsorbed on said bed. The resulting sulfurous gas mixture is cooled by indirect heat exchange (26) to a temperature of 120° to 135° C. and liquid elemental sulfur is recovered (27,54,55,101) the sulfurous gas (28) thereafter being cooled and diluted by adding cool inert gas (34) thereto to maintain the water partial pressure below the saturation vapor pressure, and the cooled diluted gas mixture is contacted with a bed of catalyst-adsorbent (32A) at a temperature of 90° to 125° C. whereby further elemental sulfur is produced by the reaction of SO 2 and H 2 S. The elemental sulfur deposits upon and/or is adsorbed on the catalyst-adsorbent and the resulting purified gas (34) is incinerated (37) and discharged (38) to the atmosphere. In one embodiment, the gas from the indirect heat exchange step (26) is contacted with catalyst-adsorbent (30A) at 120° to 135° C. before cool inert gas (34) is mixed therewith, elemental sulfur being deposited upon and/or adsorbed on the catalyst-adsorbent. Elemental sulfur is removed from the catalyst-adsorbent beds by a hot, inert purge gas (44,46) which is cooled (19B,52) for sulfur recovery. The purge gas is cooled (52) at the end of the sulfur-recovery step to cool the catalyst-adsorbent beds for a further cycle of sulfur recovery from Claus tail-gas.

What is claimed is: 
     
       1. A process for removing sulfur and sulfur compounds from a sulfurous gas containing H 2  S and SO 2  comprising the steps of: (a) passing the said gas containing H 2  S and SO 2  in a mol ratio of H 2  S:SO 2  of about 2:1 in contact with a first stage catalyst at conversion conditions to promote the reaction: 2H 2  S+SO 2  →2H 2  O+3S;   (b) separately recovering, as products from step (a), elemental sulfur and a gas depleted in sulfur moieties;   (c) cooling and diluting the gas recovered from step (b), said gas being cooled to a temperature below the freezing temperature of sulfur and being diluted by being mixed with an inert gas such that the partial pressure of water vapour in the gas is less than the equilibrium partial pressure of liquid water and water vapour at the temperature of the resulting cooled and diluted gas mixture;   (d) passing the cooled, diluted gas mixture containing H 2  S and SO 2  in contact with a catalyst-adsorbent mass in a subsequent stage to convert H 2  S and SO 2  to elemental sulfur which deposits upon and/or is adsorbed on the said mass; and   (e) discharging a gas mixture depleted in sulfur moieties.   
     
     
       2. A process as in claim 1 in which the cooling in step (c) is effected at least in part by indirect heat exchange in an indirect heat exchanger, and elemental sulfur is recovered from the thus-cooled sulfurous gas. 
     
     
       3. A process as in claim 1 in which at least part of the cooling in step (c) is effected by the addition of the inert gas at a lower temperature than the sulfurous gas. 
     
     
       4. A process as in claim 3 in which the inert gas is added to the sulfurous gas after the latter has been cooled by indirect heat exchange. 
     
     
       5. A process as in claim 2 in which the temperature of the sulfurous gas after cooling by indirect heat exchange is in the range of from 120° to 135° C. 
     
     
       6. A process as in claim 1 in which the temperature of the sulfurous gas after mixing with the inert gas is in the range of from 90° to 125° C. 
     
     
       7. A process as in claim 2 wherein the sulfurous gas is passed in contact with a catalyst-adsorbent after being cooled but before being diluted with inert gas, the catalyst-adsorbent promoting the reaction 2H 2  S+SO 2  →2H 2  O+3S, sulfur being retained on the catalyst-adsorbent and a sulfurous gas of lower sulfur moiety content being recovered and diluted with inert gas. 
     
     
       8. A process as in claim 7 wherein the temperature of the cooled but undiluted sulfurous gas contacting the said catalyst-adsorbent is in the range of from 120° to 135° C. 
     
     
       9. A process as in claim 1 in which between steps (b) and (c), the gas is passed in contact with a further catalyst-adsorbent mass to promote the reaction 2H 2  S+SO 2  →2H 2  O+3S at a temperature in the range of from 135° to 150° C. causing elemental sulfur to deposit upon and/or be adsorbed on the said further mass and a gas of reduced sulfur moiety content to pass to step (c). 
     
     
       10. A process as in claim 1 wherein elemental sulfur deposited upon and/or adsorbed on catalyst-adsorbent is recovered by intermittently interrupting the flow of sulfurous gas through the respective catalyst-adsorbent(s), and passing a hot oxygen-free purge gas in contact with the respective catalyst-adsorbent(s) to remove elemental sulfur in the vapour phase, and recovering the thus-removed elemental sulfur in a sulfur-recovery container. 
     
     
       11. A process as in claim 10 in which purge gas is passed through the respective catalyst-adsorbent(s) in a direction opposite to that of the sulfurous gas. 
     
     
       12. A process as in claim 10 in which sulfur-containing purge gas is cooled to recover elemental sulfur therefrom and then reheated to an elevated temperature and thereafter passed in contact with sulfur-containing catalyst-adsorbent. 
     
     
       13. A process as in claim 10 in which a part of the purge gas is employed to prevent access of oxygen-containing gas to elemental sulfur which has been recovered. 
     
     
       14. A process as in claim 10 in which when the flow of sulfurous gas to a respective catalyst-adsorbent mass is interrupted, the flow of sulfurous gas is diverted to an alternative catalyst-adsorbent mass.